1. Field of the Invention
The present invention relates to an angle-measuring device having a scanning device for scanning a serial code and for outputting decoded words as a position.
The present invention further relates to a line of angle-measuring devices.
2. Description of the Related Art
In many fields, absolute angle measuring instruments are increasingly used for determining the position of two bodies moved relative to one another. Absolute angle measuring instruments have the advantage over purely incrementally measuring systems that in each relative position, even after an interruption of the energy supply, correct position information can immediately be output.
The absolute position is embodied by a code. The disposition of the position information in a single code track with code elements disposed in succession in the measuring direction is especially space-saving. The code elements are disposed in a pseudo-random distribution in succession, so that a certain number of successive code elements each form one code word, which unambiguously defines the absolute position. Upon a shift of the scanning device by one single code element, a new code word is already formed, and a sequence of different code words is available over the entire circumference to be detected in absolute form. A serial or sequential code of this kind is also often called a chain code or a pseudo-random code.
As already explained in Japanese Patent Disclosure JP 57-175211 A, for determining the absolute position from the scanned code words—also called decoding—a decoding table (look-up table) is used, in which each code word is assigned one position. For assigning the absolute position to a scanned code word, the code word forms the address for the decoding table, so that the absolute position stored in memory for this code word is present at the output and is available for further processing.
These nonvolatile tables can today be designed in hard-wired in an ASIC, to make fast access possible. To procure ASICs economically, however, relatively high numbers must be produced per ASIC. The goal is therefore to minimize the great number of variants. The limits for reducing variants are reached especially in angle-measuring devices in which over the entire circumference, in every angular position, one ambiguous absolute position must be output. A client often wants different diameters of the angle-measuring devices to have a different number of absolute positions per revolution. As a rule, for each diameter a separate ASIC with an individual table, adapted to the individual code, is used for this purpose.
To enable using a scanning device for different diameters of a code disk it is proposed in Japanese Patent Disclosure JP 2005-061907 A that an individual decoding table, in the form of read-only memories (ROMs), with a complete set of values of all the code words to be decoded is provided in the scanning device for each of a plurality of variants. Based on an identification in the form of a selection signal which specifies the code to be scanned, only data from the decoding table individually assigned to that code are used during the decoding. In selecting the code, the point of departure is a generated sequence of unambiguous code words. For angle-measuring devices of smaller diameter, that is, a shorter code length, there is a complete separate decoding table whose code words are a selected section of the existing sequence of code words. The disadvantage of this angle-measuring device is that for the decoding tables and thus for the code, only very special sequences from the generated sequence of code words can be selected, and that the Hamming distance of the code words of the sequences is necessarily 1. Moreover, the space required for the plurality of complete decoding tables is relatively great.